Even as ExxonMobil was mopping up after its disgusting tar-sands oil spill in Arkansas on Wednesday, it spilled an unknown amount of unknown chemicals — possibly hydrogen sulfide and cancer-causing benzene — during an accident at a riverfront refinery in Louisiana.

The Chalmette refinery chemical spill might have gone unnoticed, except that it stank out the city of New Orleans and several nearby parishes, leading to state and federal investigations (we told you about that mysterious odor yesterday). Frankly, ExxonMobil’s track record here sucks: The same refinery spilled 360 barrels of crude oil in January.

ExxonMobil first reported releasing 100 pounds of hydrogen sulfide and 10 pounds of benzene, a volatile organic carbon compound known to cause cancer, because those amounts are the minimum required for reporting, [Coast Guard Petty Officer Jason] Screws said. But the company has since said it is unsure exactly what chemicals were involved or how much may have been released, he said.

The spill occurred as a result of a break in a pipeline connecting a drum used to store “liquid flare condensate,” with a flare on the refinery site, Screws said. He said the company measured 160 parts per million of hydrogen sulfide and 2 parts per million of benzene in the air at the site of the spill, but has not seen similar readings at the plant’s fence line or in the neighboring community.

Residents from the region inhaled chemicals caused by the spill for more than a day, leading to reports of breathing difficulties and other ailments. But the Coast Guard rushed to soothe folks, assuring them not to worry their chemical-infused heads about it. From Reuters:

“We haven’t told the refinery to shut down because we haven’t any cause for a shutdown,” [Lieutenant Lily] Zeteza said. “We’ve no indication that this is dangerous.”